There has long been a need for control of the living environment in terms of temperature and humidity. The need extends to requirements for control of the environments in which man lives and works (his shelter), and to sub-environments which house and preserve perishables. Some commercial applications of such systems include the manufacture of ice, environmental control of public and office buildings, hotels, stores and factories.
Two basic types of refrigeration systems exist in the prior art; intermittant and continuously operating systems. The continuously operating system is prevalent in modern use. Within the constraint of continuously operating refrigeration systems, there are two major sub-types. Absorbent systems (see FIG. 3) such as the Platers-Munters type (marketed by Servel, Inc.) and vapor compression systems (see FIG. 1) which widely prevail in the United States. These two sub-types share some common features. Each is provided with condenser heat exchanger 10 for converting relatively high temperature, high pressure gas to a warm liquid state, each utilizes expansion valve 14 for controlled expansion of the warm liquid to a cooler gas state and each provides evaporator heat exchanger 2 for extracting heat from a volume which is to be environmentally controlled. Heat pump versions of at least the compressor type systems are supplied with reverse cycle capability for heating the volume to be environmentally controlled. Both systems depend upon the relatively high efficiencies available when refrigerant is converted from a gas to a liquid state and from a liquid to a gas state based on the "leverage" provided by high latent heats of vaporization and condensation.
The major features of the two systems may be distinguished by inspection of FIGS. 1 and 3. Either system requires an energy input; compressor 6 requiring rotary motion, generally suppled by electric motor 5, and the absorbtion system requiring relatively high temperature source of heat 30 for desorption (FIG. 3).
The absorption system as commercialized by Servel, Inc. (FIG. 4) utilizes an absorbent, which may be water, a refrigerant, which may be ammonia, and a pressure equalizing gas, which may be hydrogen. While the partial pressure of the refrigerant varies from the high pressure side of the system to the low pressure side of the system, the introduction of hydrogen gas provides compensation providing for equalization of the total pressures on each side of the system thereby eliminating the need for a fluid pump and for a second throttling valve. The Servel, Inc., version of the absorption refrigeration system thereby avoids the use of any mechanical moving parts at all and allows for gravity and convection flow of the various liquids and vapors in the closed system.
Referring to FIG. 4, generator 200 conntains a saturated solution of ammonia absorbed in water. Heat source 202 supplies the energy required to drive ammonia gas (represented by small circles) out of the ammonia/water mixture 204. Ammonia gas is released and rises in conduit 206 to separator 208. Water is carried along by a percolation process and the mixture is separated in separator 208. The relatively ammonia-free water leaves separator 208 in conduit 210 while the liberated hot ammonia gas rises in conduit 212 to condenser 10. The gas is reduced in temperature by condenser 10 and is converted to a warm liquid at liquid trap 214. When the warm ammonia liquid passes through liquid trap 214, it sees a reduced partial ammonia vapor pressure in conduit 216 and evaporator 2. It is then able to expand at the lower pressure, becoming a relatively cold gas in evaporator 2. Heat from volume 18 is absorbed by the cold gas in evaporator 2, thereby cooling volume 18. Conduit 218 carries the warmed ammonia gas to absorbers 220 and 222.
The low ammonia content water from separator 208 is cooled in absorbers 222 and readily absorbs the warm ammonia gas from conduit 218. The ammonia-bearing water flows by gravity feed to absorber 220 and then through conduit 224 back to generator 200, there to begin the cycle over again.
Conduit 226 is filled with an inert or neutral gas such as hydrogen. The hydrogen also infiltrates the low pressure side of the system; that is, conduits 216, 218, and 222, upper portion of absorber 220 and absorbers 222. The partial pressure of hydrogen plus the partial gas pressures are additive to the total system pressure so that the total system pressure is equal in all portions of the system.
Systems utilizing dessicant beds have been described in the prior art. Some are of the type which require physical transportation of the bed, such as that described in "Solar Energy Thermal Processes", John Wiley and Sons, Duffie and Beckman, pages 341-3, and others attain intermittant operation only. Faraday demonstrated an intermittant absorber-vaporizer utilizing silver chloride and ammonia in 1824. See FIG. 2. Absorber 33 (silver chloride), which had been exposed to dry ammonia gas in one end 34 of inverted "V" test tube 36, was heated. When opposite end 38 of "V" shaped test tube 34 was cooled by insertion in water 40, liquid ammonia 42 was condensed at the cooled end. When the heat was removed from the absorber end of the test tube, Faraday noticed that the liquid ammonia in the other end boiled violently, changing back to a vapor which was reabsorbed by the silver chloride absorber. The latent heat of vaporization caused the liquid ammonia end of the test tube to be very cold.
Intermittant absorption systems were popular in the 1930's. The Trukhold refrigerator distributed by Montgomery Ward and the Icy Ball system manufactured by the Crosley Corporation were examples of this type. "(Modern Refrigeration and Air Conditioning", Althouse and Turnquist, the Goodheart-Willcox Co., Inc., 1960.)
The ammonia/water/hydrogen combination employed in the Servel system is typical of absorption systems (as contrasted to adsorption systems). There, water is used as an absorbent and absorption is of a chemical nature. In contrast, other systems utilize physical adsorbents, one of the most common being silica gel. Silica gel is a microporous inert material. It is characteristic of the material that each granual is literally full of interconnected molecular sized holes which provide an enormous amount of internal surface area. It is also characteristic of silica gel that transient adsorption rates are very rapid and the high rates of diffusive flow are attributed to strong surface diffusion phenomena. When sorbable vapors are adsorbed into high internal surface area microporous media, the diffusive flux is greatly increased by surface diffusion of mobile adsorbed films in a concentration gradient.
Another material which exhibits similar characteristics is Vycor porous glass as manufactured by the Corning Glass Works, Corning, N.Y.
E. R. Gilliland, R. F. Baddaur and H. H. Engel reported the results of an investigation of gas flow by adsorption in "Flow of Gases Through Porous Solids Under the Influence of Temperature Gradients", American Institute of Chemical Engineers Journal, 8 A.I.Ch.E.J. 530, September, 1962. Results recorded there indicate a diffusion gas flow from a cold to a hot surface through a porous solid such as Vycor. K. G. Denbigh and G. Rauman have suggested that such a flow through a thin rubber membrane may occur in the face of adverse pressure differentials, although, as well known in the art, the diffusion rate is as much as 1,000 times slower than the diffusion through Vycor porous glass. 210A Proceedings of the Royal Society (London) 518 (1951). Gilliland et al, supra, at 530, stated, "both the gas phase and surface flows are from the cold end to the hot end of the porous solid." They suggested, in conclusion, supra, at page 535, "Isobaric permeabilities of the pure adsorbed gases investigated are considerably higher than the values predicted from correlation based on free-molecular flow data. The higher rates of flow are attributed to a net migration of adsorbed gas along the surface of the pores." This high flow rate migration phenomona is not noticable in non-hygroscopic materials.
The phenomona reported by Gilliland et al, supra, may be better understood by means of the following: Since the adsorption process is exothermic upon occurance, the application of heat at one adsorbent interface provides the endothermic desorption of sorbate molecules and the rejection of heat at the opposing interface provides the exothermic adsorption of the sorbate molecules. The addition of sorbate molecules at the cold interface and the deletion of sorbate molecules at the hot interface establishes a concentration gradient in the adsorbent that drives the diffusion of adsorbed vapors from cold to hot. This phenomenon is more generally described by the thermodynamics of irreversible processes and is closely analogous to the thermoelectric effect. In any overview, the predominate driving forces are provided for by the concentration gradients.
The conditions of the experiment described by Gilliland et al, supra, were such that the pressure was held constant and was far below the saturation pressure of the vapors investigated.
Various means have been devised to provide heating or cooling from the solar energy source. One of the major problems which limits commercial feasibility of these systems is a very high initial cost as the result of schemes used to raise the effective temperature of the energy source by some sort of heat concentration means. The temperature increase has been found to be desirable and necessary in prior art systems in order to provide more efficient heat transfer characteristics in the heating/cooling systems.
Prior art cooling systems may be classified as evaporative or refrigerant systems. The evaporative systems are very ineffective unless operated in dry climates. Mohr's U.S. Pat. No. 2,202,019 is an example of such a system. Altenkirch's U.S. Pat. No. 2,138,691 teaches the use of silica gel and/or wood shavings as adsorbents used for the removal of moisture from air.
In solar engergized systems, some means must be supplied to provide a source of heat energy at those times when solar energy is unavailable or is in short supply, i.e., at night or on overcast days. Lof's U.S. Pat. No. 2,680,565 teaches the use of a heat storage bed made of "a loose or spaced solid, such as sand, gravel or stacked brick, but which may be a fluid, such as tar, oil, water or the like." (U.S. Pat. No. 2,680,565, Col. 6, lines 40-42.) The storage bed may be charged with heat energy at those times of relatively bright sunlight and may be drawn upon as a source of heat energy when adaquate sunlight is not available.
Solar energy heating/cooling systems have been slow to receive public acceptance because of their complexity and high initial cost and because of the general tendency for the systems to be bulky and inefficient.